In the field of ophthalmic instruments, non-contact tonometers are well known for measuring intraocular pressure. Early non-contact tonometers, such as that introduced by Bernard Grolman in U.S. Pat. No. 3,585,849, measured intraocular pressure by directing an increasing force air pulse at the cornea to deform the cornea inward from an original convex state through a first flattened or “applanated” state to a concave state, and allowing the cornea to return outward from the concave state through a second applanated state to its original convex state with disintegration of the air pulse. Deformation of the cornea was monitored by an infra-red emitter and detector arranged on opposite sides of a test axis aligned with the cornea, such that the detector would receive light after reflection by the cornea and generate a signal characterized by first and second signal peaks corresponding to the moments of inward and outward applanation. The deformation signal was analyzed in conjunction with an increasing ramp signal of force-versus-time associated with generation of the air pulse by a solenoid-driven pump mechanism, whereby the time interval required to achieve inward applanation was used as a correlate of intraocular pressure.
Taking advantage of improvements in miniaturized sensor technology, more recent non-contact tonometers have abandoned reliance on a time interval correlate, and instead provide a pressure sensor within a plenum chamber of the pump mechanism to directly measure plenum pressure as a function of time during corneal deformation. The pressure signal from the pressure sensor is analyzed with the opto-electronically obtained deformation signal to determine intraocular pressure. See, for example, U.S. Pat. No. 7,481,767 to Luce.
The observation that a pressure differential exists between a plenum pressure associated with inward or first applanation and a pressure associated with outward or second applanation (referred to as “corneal hysteresis”) has led to improvements in the accuracy of the intraocular pressure measurement and derivation of supplemental information about biomechanical characteristics of the corneal tissue. In this regard, see U.S. Pat. Nos. 6,817,981; 6,875,175; 7,004,902; and 7,481,767.
Nevertheless, it has long been recognized that a series of intraocular pressure measurements on a given eye will vary due to variability in the physical measurement process, such as slight differences in alignment of the instrument relative to the eye and randomly timed blinking by the test subject. Consequently, it has been accepted practice to perform a plurality of measurements on a given eye and to average the results. Also, it is known to discard what are perceived to be “outlying” intraocular pressure values from a set of measurements on an eye prior to averaging the remaining intraocular pressure measurement values.
Historically, the corneal deformation signal has always been analyzed in conjunction with a second metric, either a time interval or plenum pressure, to determine intraocular pressure and/or biomechanical characteristics of the corneal tissue. The corneal deformation signal has never been analyzed independently to yield information about the eye or about the physical measurement process giving rise to corneal deformation.